This invention relates generally to nuclear reactors and more particularly to a storage arrangement for nuclear reactor fuel assemblies.
A nuclear reactor produces heat by the fissioning of a fissile material fuel. The fissile material is located within fuel elements, and a plurality of fuel elements are assembled into a fuel assembly. A plurality of fuel assemblies comprise a nuclear core.
During the course of reactor operations, the fuel will become depleted, or spent, and must be removed from the nuclear reactor. The spent fuel assemblies are generally removed during refueling operations, which typically occur approximately once a year. During the refueling operations, the spent fuel assemblies are removed from the reactor core, and new fuel assemblies are inserted into the reactor core. After being removed from the core, the spent fuel assemblies are generally placed in storage to await further disposition. The storage for the spent fuel containers is typically a separate fuel storage tank, although, in some reactor installations, the storage is within the nuclear reactor pressure vessel outside the core. In either location, the spent fuel assemblies are kept in storage until they are either removed to permanent storage locations or retrieved for fuel reprocessing.
Even after removal from the core, the spent fuel assemblies continue to generate both heat and fission products, namely gamma and beta radiation, and some neutrons. Therefore, while in storage, means must be provided to cool the spent fuel. Additionally, the spent fuel must be prevented from becoming critical; that is, means must be provided to prevent the spent fuel from propagating a chain-reaction in the storage tank where means for controlling such reaction are not present.
Various types of means have been utilized to prevent the initiation of a chain-reaction. A neutron-absorbing material, such as boron, has been placed between the fuel elements to absorb the excess neutrons produced by the fuel which might otherwise initiate a chain-reaction. Additionally, the containers into which the spent fuel assemblies are placed are generally physically separated from each other so as to prevent the neutrons from one fuel assembly from reacting with the fuel of another fuel assembly. Both of these methods require relatively large distances between adjacent fuel assemblies in the storage container.
One of the problems with the aforementioned storage systems involves the length of time the spent fuel assemblies must remain in storage. At this time, there is no licensed method for the final disposition of the spent fuel, and none which will be available in the near future. Therefore, all the spent fuel which must be removed from the reactor core must be placed in this storage for an indeterminate period of time. A nuclear reactor installation may be required to store, for example, the equivalent of four or five complete cores. The temporary storage of this large number of fuel assemblies within an acceptable facility requires a large financial expenditure.
Providing adequate storage space within an adequate container for the large number of assemblies requires a large volume within the storage container. One method of attempting to reduce the radial space requirements is to use multiple storage levels, wherein the fuel assemblies are stacked one atop the other, with the fuel in any one level being at a common elevation. Although this multiple stacking arrangement may be more practical, large amounts of space in the storage container cannot be utilized for fuel storage because these spaces are occupied by the sections of the fuel assembly duct beyond the ends of the active fuel wherein fuel is not present. What is required is an arrangement wherein the space occupied by the fuel assembly ducts beyond the active fuel is utilized for active fuel storage in an arrangement that prevents the initiation of a chain-reaction.